Now a description is made of a conventional LCD projector by referring to FIG. 10.
As shown in the plan of FIG. 10, the conventional LCD projector using three LCD panels, comprises a light source unit 1, a color separating/recombining unit 2, a reflective light modulating unit 3 consisting of three LCD panels 31, 32, 33 that modulate separated color components to convert into image lights of each color, and a projection optical system 4 including a projection lens.
The light source unit 1 comprises a lamp 11a, a reflector 11b and polarization conversion means (not shown) that converts light from the lamp 11a into polarized light having a given polarization direction, so that the light source unit 1 may emit light polarized in a particular polarization direction. The color separating/recombining unit 2 is provided with two dichroic mirrors 21, 23 and one reflective mirror 22 to separate light incident from the light source unit 1 into three primary color components. In this projector, light emitted from the light source unit 1 is converted into s-polarized light by the polarization conversion means.
Of the light emitted from the light source unit 1, for example, s-polarized light of a red component reflects perpendicularly off a first dichroic mirror 21 and a blue component and a green component pass through the first dichroic mirror 21 and the separated red component enters a first polarizing beam splitter (hereafter referred to as PBS), namely a PBS for red 24.
The s-polarized light of the blue component and the s-polarized light of the green component, which pass through the first dichroic mirror 21, reflect perpendicularly off the reflective mirror 22 and enter a second dichroic mirror 23. The blue component passes through the second dichroic mirror 23, while the green component reflects off the second dichroic mirror 23.
The s-polarized light of the blue component, which passes through the second dichroic mirror 23, enters a second PBS, namely a PBS for blue 25, while the s-polarized light of the green component, which reflects perpendicularly off the second dichroic mirror 23, enters a third PBS, namely a PBS for green 26.
The PBS for red 24 comprises a polarized light separating surface 24a inside, which s-polarized light reflects perpendicularly and p-polarized light passes through. After being emitted from the first dichroic mirror 21, the red component reflects perpendicularly off the PBS for red 24 toward a first LCD panel 31. On the basis of image information for red, the LCD panel 31 rotates the polarization direction of the red light 90 degrees, which entered the part to be displayed in red, that is, the LCD panel 31 converts s-polarized red light into p-polarized red light and reflects it. The p-polarized red image light, which is reflected by the LCD panel 31, passes through the PBS for red 24 and enters a dichroic prism for recombining 27.
The PBS for blue 25 comprises a polarized light separating surface 25a inside, which s-polarized light reflects perpendicularly and p-polarized light passes through. After passing through the second dichroic mirror 23, s-polarized light of the blue component reflects perpendicularly off the PBS for blue 25 toward a second LCD panel 32. P-polarized blue image light, which is reflected by the LCD panel 32, passes through the PBS for blue 25 and enters the dichroic prism for recombining 27 from the opposite side where the red image light enters.
The PBS for green 26 comprises a polarized light separating surface 26a inside, which s-polarized light reflects perpendicularly and p-polarized light passes through. After reflecting off the second dichroic mirror 23, s-polarized light of the green component reflects perpendicularly off the PBS for green 26 toward a third LCD panel 33. P-polarized green image light which is reflected by the LCD panel 33, passes through the PBS for green 26 and enters the dichroic prism for recombining 27 from a side orthogonal to sides where the red image light and the blue image light enter.
The inside of the dichroic prism for recombining 27 comprises two dichroic surfaces 27a, 27b intersecting with each other at right angles. The first dichroic surface 27a that the blue image light perpendicularly reflects off and the red image light and the green image light pass through, recombines the blue image light and the green image light. The second dichroic surface 27b that the blue image light and the green image light pass through and the red image light perpendicularly reflects off. As a result, the image lights of these three primary colors are recombined.
As shown in the plan of FIG. 11, another conventional LCD projector using three LCD panels comprises a polarizing plate 12 transmitting only p-polarized light of white light emitted from a light source unit 1 provided with a light source 11 comprising a lamp 11a and a reflector 11b. 
A color separating/recombining unit 5 comprises a narrow-band retardation plate 51, which rotates a polarization direction 90 degrees, of p-polarized light having predetermined wavelength, for example only a red component, to convert into s-polarized light, a PBS for separating red 52, a PBS for red 53, two narrow-band retardation plates 54, 56, which rotate a polarization direction of only p-polarized light of the blue component by 90 degrees to convert into s-polarized light, a PBS for separating/recombining blue and green 55, and a PBS for recombining 57.
Although white light emitted from the lamp 11a is emitted as p-polarized white light by the polarizing plate 12, the first narrow-band retardation plate 51 converts only red component into s-polarized light.
The s-polarized light of the red component reflects perpendicularly off the PBS for separating red 52 and the PBS for red 53, and goes to LCD panel for red 31.
P-polarized light of the blue component, which passes through the PBS for separating red 52, is rotated its polarization direction 90 degrees to be converted into s-polarized light by the second narrow-band retardation plate 54. The s-polarized light of the blue component reflects perpendicularly off the PBS for separating/recombining blue and green 55 to enter LCD panel for blue 32.
P-polarized light of the green component, which passes through the PBS for separating red 52, passes through the second narrow-band retardation plate 54 and the PBS for separating/recombining blue and green 55 to enter LCD panel for green 33.
These reflective LCD panels 31, 32, 33 rotate the polarization direction 90 degrees, of light which is incident to pixels to be displayed on the basis of image information on each color, and reflect the light in the opposite direction from which the light entered. Light, which is incident to pixels and not displayed, is reflected by the LCD panels in the opposite direction from which the light entered without changing its polarization direction.
Since the red image light reflected by the LCD panel for red 31 is p-polarized light, the red image light passes though the PBS for red 53 and the PBS for recombining 57 to enter a projection optical system 4. Since the blue image light reflected by the LCD panel for blue 32 is p-polarized light, after passing through the PBS for separating/recombining blue and green 55 and being converted to s-polarized light by the third narrow-band retardation plate 56 again, the blue image light reflects perpendicularly off the PBS for recombining 57 to enter the projection optical system 4.
Since the green image light reflected by the LCD panel for green 33 is s-polarized light, after reflecting perpendicularly off the PBS for separating/recombining blue and green 55 and passing through the third narrow-band retardation plate 56, the green image light reflects perpendicularly off the PBS for recombining 57 to enter the projection optical system 4.
In the conventional LCD projector of FIG. 10, the PBS for red 24, the PBS for blue 25, the PBS for green 26 and the dichroic prism for recombining 27 are each formed in a cube. In consideration of diffusion of light occurring between the light source unit 1 and the projection optical system 4, one side of each surface of these cubes should be longer than one side (or longer side) of each LCD panel 31, 32, 33. The two dichroic mirrors 21, 23 and the total reflective mirror 22 should be formed in rectangular so that the projected light in an incident direction and in a reflecting direction becomes a square bigger than an area of each LCD panel 31, 32, 33.
By so designing, the color separating/recombining unit 2 requires an area more than nine times larger than one of the PBS 24, 25, 26 or the dichroic prism 27, which is unfavorable in miniaturizing the color separating/recombining unit 2 and LCD projector using the unit. Also, since the path length for each color component between each LCD panels 31, 32, 33 and the projection optical system 4 is long, an aperture size of the projection optical system 4 should be big enough, which makes miniaturization of the LCD projector difficult.
In the conventional LCD projector in FIG. 11, the color separating/recombining unit 5 would be smaller than that in FIG. 10 because four PBSs 52, 53, 55, 57 are arranged in two lines horizontally and vertically on a two-dimensional view.
However further miniaturization of the color separating/recombining unit 5 has been considered impossible because the idea that color components in the color separating/recombining unit 5 must be separated and recombined in a plan is deeply rooted.
Considering these circumstances, we successfully achieved a remarkably small color separating/recombining unit as compared with the conventional ones by three-dimensionally separating and recombining color components through various trials and errors, and finally completed our invention.
The present invention has an object to provide a remarkable miniature color separating/recombining means and a compact LCD projector.